1. Field of the Invention
The present invention relates to a dielectric resonator, a dielectric filter, and a duplexer for use in the microwave or millimeter wave range and also to a communication device using such an element.
2. Description of the Related Art
In recent years, with the increasing popularity of mobile communications systems and multimedia, there are increasing needs for high-speed and high-capacity communications systems. As the quantity of information transmitted via these communications systems increases, the frequency range used in communications is being expanded and increased from the microwave range to the millimeter wave range. Although TE01xcex4-mode dielectric resonators, which are widely used in the microwave range, can also be used in the millimeter waver range, extremely high accuracy is required in forming resonators because the dimensions of the cylindrical dielectric of the resonator, which determine the resonant frequency of the resonator, become very small in the millimeter wave range. In the case where a filter for use in the millimeter wave range is constructed using TE01xcex4-mode dielectric resonators, extremely high positioning accuracy is required when TE01xcex4-mode dielectric resonators are disposed at properly spaced locations in a waveguide. Furthermore, the resonance frequency of each resonator should be adjusted precisely. It is also required that coupling among dielectric resonators be precisely adjusted. However, a very complicated structure is required to perform precise adjustment.
The applicant for the present invention has proposed, in Japanese Patent Application No. 7-62625, a dielectric resonator and a bandpass filter which does not have the above problems.
FIGS. 10A and 10B illustrate the structure of the dielectric resonator disclosed in the patent application cited above, wherein only the essential parts are shown in the figure. In FIGS. 10A and 10B, reference numeral 3 denotes a dielectric substrate having a particular relative dielectric constant. Electrodes 1 and 2 are formed on opposite principal surfaces of the dielectric substrate 3 such that each electrode has a circular-shaped electrodeless areas 4 or 5 whose diameter is properly determined. Conductive plates 17 and 18 are disposed at opposite sides of the dielectric substrate 3 so that they are spaced by a proper distance from the dielectric substrate 3. In this structure, a resonator region 60 with a cylindrical shape is formed in the dielectric substrate 3 and it acts as a TE010-mode dielectric resonator.
In the above dielectric resonator having the structure including electrodes having electrodeless areas with substantially the same shape which are formed on opposite principal surfaces of the dielectric plate disposed between the two conductive plates spaced from each other, spurious waves in a TE mode are generated between the respective electrodes on the principal surfaces of the dielectric plate and the corresponding conductive plates, and the spurious waves propagate in the spaces between the principal surfaces of the dielectric plate and the conductive plates. The spurious waves are reflected by a cavity wall and thus standing waves are generated. This means that resonance associated with such standing waves occurs.
If such TE-mode spurious waves are generated and propagate in the spaces between the respective principal surfaces of the dielectric plate and the conductive plates, energy of TE010-mode resonance which is essential in this dielectric resonator is partially transferred to energy of the spurious waves, and thus the unloaded Q (Qo) becomes low and degradation occurs in the characteristics in the frequency ranges out of the passband of the bandpass filter.
One technique for constructing a dielectric resonator and a bandpass filter which do not have the above problems has been proposed by the applicant for the present invention as disclosed in Japanese Patent Application No. 8-54452.
It is an object of the present invention to provide a dielectric resonator, a dielectric filter, a duplexer, and a communication device using such an element, in which the above-described problems are prevented in a different manner from that employed in Japanese Patent Application No. 8-54452.
According to an aspect of the present invention, there is provided a dielectric resonator including electrodes formed on opposite principal surfaces of a dielectric plate, electrodeless areas having substantially the same shape being formed in the respective electrodes such that the electrodeless areas are located at positions corresponding to each other on the opposite principal surfaces of the dielectric plate, a region between the electrodeless areas serving as a resonance region, the electrodeless areas being surrounded by a cavity formed inside a conductive case, the dielectric resonator being characterized in that: the dimensions of the cavity are determined so that the cutoff frequency of the cavity is higher than the resonant frequency of the resonance region and so that the size of the cavity is greater than the outer size of the electrodeless areas thereby ensuring that generation of spurious waves in a space between the electrodes on the principal surfaces of the dielectric plate and the inner wall of the cavity.
In the above dielectric resonator, the cavity is preferably formed into a cylindrical shape with an inner diameter 2a which satisfies the condition a less than c/(3.412of) where fo is the resonant frequency of the resonance regions and c is the velocity of light.
When the cavity is regarded as a circular waveguide having a radius a, the lowest-order mode of the circular waveguide is TE11, and its cutoff wavelength xcexc. is given by xcexc=3.412a. Therefore, if the radius an is selected such that a less than c/(3.412fo) where fo is the resonant frequency of the resonant region and c is the velocity of light, then the TE11 wave is cut off and thus the propagation of the TE11 wave in the cavity is suppressed.
The cavity may also be formed into a rectangular shape with a width a which satisfies the condition a less than c/(2fo) where fo is the resonant frequency of the resonance regions and c is the velocity of light.
When the cavity is regarded as a rectangular waveguide, the lowest-order mode is TE10, and the cutoff frequency xcexc is given by xcexc=2a. Therefore, if the width an is selected such that a less than c/(2fo) where fo is the resonant frequency of the resonant region and c is the velocity of light, then the TE10 wave is cut off and thus the propagation of the TE11 wave in the cavity is suppressed.
According to another aspect of the present invention, there is provided a dielectric filter including electrodes formed on opposite principal surfaces of a dielectric plate, a plurality of electrodeless areas having substantially the same shape being formed in the respective electrodes such that the electrodeless areas on one principal surface of the dielectric plate are located at positions corresponding to the positions of the respective electrodeless areas on the other principal surface on the opposite side, the respective regions between the electrodeless areas serving as resonance regions, said electrodeless areas being surrounded by a cavity formed inside a conductive case, the dielectric filter further including a signal input part and a signal output part which are each coupled with an electromagnetic field in the vicinity of any of the plurality of resonance regions, the dielectric filter being characterized in that the width of the cavity at the boundary part between adjacent electrodeless areas is determined so that the cutoff frequency associated with the boundary becomes higher than the resonant frequency of the resonant regions, thereby ensuring that generation of spurious waves in a space between the electrodes on the principal surfaces of the dielectric plate and the inner wall of the cavity. Thus the resultant dielectric filter is excellent in that large attenuation is achieved in the frequency ranges outside the passband and that spurious waves are suppressed.
The coupling between adjacent resonators formed in the corresponding resonance regions can be adjusted by properly selecting the width of the boundary part of the cavity.
In this dielectric filter, the cavity surrounding the electrodeless areas is preferably formed into a cylindrical shape, and the width e of the boundary part of said cavity is determined such that e less than 2/(2fo) where fo is the resonant frequency of the resonance regions and c is the velocity of light.
The cavity acts as a waveguide in which the cutoff frequency at the boundary part is given by c/(2fo). Therefore, if the width e is selected such that e less than c/(2fo), propagation of spurious waves through the boundary part is suppressed.
According to still another aspect of the invention, there is provided a duplexer characterized in that a dielectric filter comprising a dielectric resonator according to any of aspects of the invention and further comprising a signal input part and a signal output part or a dielectric filter according to the above aspect of the invention is used as a transmitting filter or a receiving filter or both receiving and transmitting filters, the transmitting filter being disposed between a transmission signal input port and an input/output port, the receiving filter being disposed between a received signal output port and the input/output port.
According to still another aspect of the present invention, there is provided a communication device characterized in that it includes an RF circuit having a dielectric resonator according to any of aspects of the invention, a dielectric filter according to any of aspects of the invention, or a duplexer according to the aspect of the invention.